This invention relates to a radon detector and, in particular to a small, compact instrument for measuring the concentration of both attached and unattached radon progeny in an atmosphere.
Radon is a disintegrating by-product of radium that is oftentimes found trapped within the ground in many geographical locations. As an inert gas, radon has a relatively low chemical interaction rate and, as a result, it can diffuse through soil and thus accumulate within buildings or the like which are situated over areas of high radon concentration. Continual human exposure to certain levels of radon progeny have been shown to be harmful to one's health. The progeny of radon can be divided into two general groups or factions. The first fraction involves what is referred to as "unattached" progeny wherein airborne progeny is carried along by ultra fine aerosol particles, each having a size of 10 nm or less. The second fraction is referred to as "attached" progeny wherein the progeny is attached to a micron size or larger dust particle.
Radon progeny generally enters into a home or dwelling through the basement or lower floors where it can accumulate to critical levels. The principle health threat from radon comes from the fine unattached progeny which have greater mobility over its attached counterpart and is more easily deposited on human lungs. The health risk posed exposure to unattached progeny is further increased in buildings where smokers reside. Similarly, homes having gas ranges, air purifiers and the like which can significantly alter the aerosol distribution in the atmosphere also tend to increase the concentration of unattached progeny fractions and thus increase the risk factor. It has therefore become extremely important to be able to detect both the presence of attached and unattached progeny fractions in an enclosed structure such as a home so that the risks involved can be accurately assessed.
In U.S. Pat. No. 3,922,555 there is disclosed a portable device for use in uranium mines that is able to detect the presence of alpha particles emitted by two daughters of radon. The device has a hollow housing and a small fan for drawing atmospheric air into the housing. A filter is positioned within the chamber for capturing and retaining progeny particles found in the air flow. A pair of screens are placed between the filter and a radiation detector. One screen is arranged to pass alpha particles emitted by a first progeny, radium A, while blocking radiation emitted by other progenies. The second screen is arranged to pass alpha particles emitted by a second progeny, radium C', while similarly blocking radiation emitted by other progenies. The detector uses a film that is capable of recording tracks of alpha particles that pass through the two screens. The number or density of the tracks recorded over the two screened areas are then measured and the concentration of the A and C' progenies present in the atmosphere computed from these measurements.
A device for testing air filtration systems for removing radon progeny from an air flow is also disclosed in U.S. Pat. No. 3,614,421. This type of filtration system is intended to operate as a high efficiency device for capturing and retaining most, if not all the radon progeny found in a moving air stream. Radon progeny detectors are placed on the upstream and downstream side of the filter system which provide information concerning the concentration of radon progeny approaching and leaving the filter area. Each detector contains a capture chamber connected to a vacuum pump which draws a sample flow of atmosphere through the filter which captures the radon progeny. Alpha particles emitted by the trapped radon progeny are recorded as tracks in a cellulose based film. Valuable information concerning the effectiveness of the filtering system is thus gained by simply comparing the measured concentration of tracks recorded by the upstream detector with those recorded by the downstream detector.
Passive radon detectors operating on the same general principles as those described in the above noted patents, are also set forth in U.S. Pat. Nos. 4,704,537; 4,518,860; 4,385,236; and 3,665,194.
Although all the above noted detectors are fully capable of detecting and measuring levels of decaying by-products, such as radon, these devices are nevertheless not capable of distinguishing between the concentrations of unattached and attached fractions. Accordingly, the ratio of progeny present to radon gas (F) in a sample of air, as well as the ratio of the unattached progeny to the total progeny (f) can only be inferred from average measurements. These inferred levels, for an average home, may vary significantly so that the risk factor is correspondingly either over or under estimated by a considerable amount.